1. Field of the Invention
The present invention relates to a LED (light emission diode) driver, and more particularly to a control circuit for controlling the LED.
2. Description of Related Art
The LED driver is utilized to control the brightness of LED in accordance with its characteristic. The control of the LED is to control the current that flow through the LED. A higher current will increase intensity of the brightness, but decrease the life of the LED. FIG. 1 shows a traditional approach of the LED driver. The voltage source 10 is adjusted to provide a current ILED to LEDs 20˜25 through a resistor 15. The current ILED can be shown as equation (1),
                              I          LED                =                              V            -                          V                              F                ⁢                                                                  ⁢                20                                      -                          V                              F                ⁢                                                                  ⁢                21                                      -            …            -                          V                              F                ⁢                                                                  ⁢                25                                                          R            15                                              (        1        )            wherein the VF20˜VF25 are the voltage drop of the LEDs 20˜25 respectively. The drawback of the LED driver shown in FIG. 1 is the variation of the current ILED. The current ILED is changed in response to the change of the voltage drop of VF20˜VF25, in which the voltage drop of VF20˜VF25 will be change due to the variation of the production and operating temperature. The second drawback of the LED driver shown in FIG. 1 is the power consumption of the resistor 15. FIG. 2 shows another traditional approaches of the LED driver. A current source 35 is connected in series with the LEDs 20˜25 for providing a constant current flow through the LEDs 20˜25. However, the disadvantage of this circuit is the power loss of the current source 35, particularly as the voltage source 30 is high and the LED voltage drop of VF20˜VF25 are low. The objective of the present invention is to provide a LED driver for reducing the power consumption and achieving higher reliability. The second objective of the present invention is to develop a high efficiency method for controlling the brightness of the LED.